Such cutting tool assemblies have long been known particularly where the cutting head is replaceably retained in or on a tool holder by screw clamping. With most of such known assemblies handling difficulties are often encountered when removing or replacing the cutting head, seeing that this usually involves loosening, removal and screw insertion of the clamping screw, whilst at the same time holding the cutting head. Additionally, with many types of cutting tools the clamping screw is very often called on to absorb a significant portion of the torque load exerted on the cutting head when in use.
These problems become particularly acute in the case of miniature cutting tools. Such miniature cutting tools are in general used for internal and external machining such as, for example, in turning, milling, grooving, drilling, etc. These tools are in general constituted by a hollow shank of which a front, elongated shank portion is of relatively small diameter (for example, 6 to 14 mm).
In general, the cutting head is releasably screw coupled to a front leading end of the front shank portion. Such a tool is disclosed, for example, in International Application Serial No. WO 94/23874.
With such an arrangement, the coupling screw must be capable of withstanding the torque load arising out of the axial pre-load originating with the screw mounting of the cutting head on the front shank portion. Furthermore, with increasing reduction in size of the cutting head, smaller and smaller coupling screws have to be used, with consequent danger of the screw being sheared off as the torque forces increase. In addition, the use of such small screws requires equally small turning keys. It will be realized that with such small keys the tightening force which can be exerted is very limited. Such small keys are also subject to easy fracture and wear.
Replacement and fitting of the cutting head involves handling of the screw which can be very diminutive, and it can readily fall and get lost, as is also the case with the very small cutting heads and tightening keys.
It has been proposed in DE 3402547 to produce a cutting tool wherein the cutting head continues to be screw coupled to the front tool shaft but wherein, by virtue of the fact that radially formed ribs on an undersurface of the cutting head mate within correspondingly radially formed grooves on the adjacent surface of the front shaft, the rib and groove arrangement take up the torque forces and this, of course, reduces the stress on the coupling screw. Nevertheless, the use of such a coupling screw, and especially a diminutive screw, is subjected to the disadvantages referred to above.
In an alternative embodiment of a cutting tool disclosed in DE 4026651, the cutting head is fitted to the tool holder via an elongated draw rod which extends through the front shaft portion into a hollow region in the rear shaft portion and is effectively keyed against rotation with respect to the shaft. The draw rod is coupled to a gripping mechanism located in the rear shaft portion and accessible therefrom for retaining the cutting head against the front shaft portion or for displacing it away therefrom. Here, torque forces are taken up by the relatively narrow draw rod adjacent the front end of the shaft, whilst retention and release of the cutting head is by way of a relatively complicated gripping mechanism.
It is an object of the present invention to provide a new and improved cutting tool assembly wherein the above-referred-to difficulties and disadvantages in coupling and uncoupling the cutting head to the tool holder are significantly reduced.